1. Field of the Invention
This invention relates to a novel process for preparing 2,2-dimethoxy-2-phenylacetophenone.
2,2-Dimethoxy-2-phenylacetophenone is a compound represented by the formula ##STR1## which is widely used as a light sensitizer in polymerization reaction systems in which polymerization is initiated by radical generation with ultraviolet irradiation. Applications of the ultraviolet-cured polymerization-reaction system widely include lithography, flexography, UV-curing ink for silk screens, solder resist, etch resist for materials in electronic industry, finish paints for wood products, metal coating and other various fields.
2. Description of the Prior Art
Heretofore, 2,2-dimethoxy-2 phenylacetophenone has been produced by such processes as
a) reaction of benzil with thionyl chloride and methanol (Japanese Patent Publication No. 56456/1982),
b) reaction of benzil with methyl orthoformate or methyl orthosilicate (Japanese Patent Application Laid-Open-to-Public No. 128729/1981) and
c) reaction of benzil with dimethyl sulfate and sodium methylate (Japanese Patent Application Laid-Open-to-Public No. 125151/1977). The above process a) in which cheap starting materials can be used is associated with operational problems to be solved including the need for complete trapping of sulfurous acid gas formed as a byproduct in addition to the need for distilling off a resulting dimethyl sulfite under reduced pressure. The process b) cannot be considered as economically advantageous because expensive methyl orthoformate or methyl orthosilicate is used. Although the reaction in the process c) proceeds under moderate reaction conditions with materials available at a relatively low cost, i.e. dimethyl sulfate and sodium methylate, the process is disadvantageous in that dehydrative distillation of a dioxane-water azeotrope is required in the dioxane-recovery step due to a large amount of dioxane used as a solvent and a large amount of water added to the reaction mixture.
Moreover, it is common with these prior art techniques that the starting benzil occasionally remains unseparated in the 2,2-dimethoxy-2-phenylacetophenone product, and that use of such impure 2,2-dimethoxy-2-phenylacetophenone occasionally brings yellow staining on the polymer product due to the presence of the benzil.
Therefore, a process for the production of 2,2-dimethoxy-2-phenylacetophenone is needed which eliminates the disadvantages necessarily associated with the conventional process a), b) or c).
The process c) per se may be an acceptable one except for the separation and recovery procedures for the dioxane used because the reaction can proceed under moderate reaction conditions using dimethyl sulfate and sodium methylate both of which are available at a relatively low cost. The reason why dioxane is employed in the process c) as a solvent lies in that benzil is soluble in this solvent and that, therefore, the reaction proceeds in a homogeneous phase. After completion of the reaction dimethyl sulfate and sodium methylate as well as sodium monomethyl sulfate byproduct are to be separated by washing with water, and thus, the dioxane is necessarily diluted with the washings. If water-immiscible solvents such as hydrocarbon solvents could be used, the washing with water and the subsequent solvent-separation step would become much simpler. However, as benzil is hardly soluble in hydrocarbon solvents, a reaction using such solvents will proceed in a heterogeneous phase and the progress of acetalization will be much inhibited. Thus, development of techniques for smoothly progressing the reaction using hydrocarbon solvents is desired.